Processes of producing consolidated powdered metallurgical articles are well known. For example, U.S. Pat. No. 4,747,999 to Hasselstrom describes a powder metallurgical method of producing a consolidated powder metallurgical article using a particulate pressure medium which is preheated in a special container, in a "fluidized" bed to the forging temperature, then transferred to an outer mold and pressure is applied to the pressure medium by means of a pressure tool. Unfortunately, this method requires a pressure medium and one has to have a fluidized bed and equipment to preheat this pressure medium then transfer its contents to an outer mold.
This method of manufacture does not provide a means to control shrinkage or physical dimensions in the ceramic mold and/or consolidated article, but requires a shrinkage compensated model. This process would not allow existing parts, dies, molds and patterns with finished dimensions to be candidates for the original model. This requirement of producing an oversized model increases steps and labor costs.
Inter-particle friction of this pressure medium has additional drawbacks. This inter-particle friction is described in UK patent 2140825A, in which a method is described to reduce this friction and inconsistent pressure. Inter-particle friction requires this process to use increased pressure or heat to consolidate the powder metal article requiring larger equipment and energy costs. This prior art process has the additional limitations of requiring the crushing of the ceramic mold to remove finished articles. This removal method would not allow the ceramic mold material to be removed from internal passageways or cavities. This severely limits part configuration possibilities.
Other prior art uses heat and pressure to further increase the density of a pre-consolidated powder metal article requiting metal dies machined to desired part geometry. This die is loaded with metal powder and compressed in the range of 20,00-100,000 p.s.i. at room temperature. The article is then ejected from the die and transferred to a furnace and sintered to increase density or particle bonding. The article is then transferred to a forging die and is surrounded by a particulate pressure medium, heat and applied pressure transfers through this medium to further consolidate the metal article to high density.
The disadvantages of this prior art requires numerous steps and equipment to produce a powder metal article of complex shape, high density and dimensional accuracy. Cold compaction pressures of 20,000-100,000 p.s.i., followed by sintering requires expensive powder compacting equipment, metal tooling and a sintering furnace to produce a pre-consolidated metal part. The additional steps of placing a pressure medium in a forging die, positioning the metal article upon this medium, covering the metal article with additional medium and heating the pressure medium and metal article to forging temperature to consolidate the metal article to high density increases labor, material and energy costs.
An additional drawback of requiring a pressure medium is interparticle friction. Compressing the medium causes non-uniform pressure on the metal article being consolidated, which results in distortion and loss of dimensional accuracy. U.S. Pat. Nos. 4,539,175 and 4,501,718 give reference to reducing distortion. The loss of dimensional accuracy requires secondary machining of the article to final its dimensions which increases costs. This pressure medium has the additional disadvantages of increasing surface area and volume, requiring larger forging dies, and requiring additional pressure and/or heat. The results being larger equipment is required to consolidate a metal article and an increase in equipment, energy and labor costs.
U.S. Pat. No. 4,041,123 discloses a method of producing a powder article which requires producing a pre-consolidated article by mixing a ceramic powder and water to form a slurry which is casted into a porous mold. This pre-formed body has a void content of 30-60%. A pressure medium, heat and applied pressure are used to further consolidate the article to higher density. This method has several disadvantages. Slip casting requires a porous mold to be fabricated from an original article or rubber mold and requires water contents of 40-70% which leaves a particulate density of only 30-60% by weight, and tremendous shrinkage results both when drying and in final consolidation. In addition, the density of a slip cast part varies. This is caused by larger, heavier powder particles settling at the bottom of the mold causing density gradients and resulting in non-uniform shrinkage which is well known in the art. This distortion is further increased by the use of a particulate pressure medium. Interparticle friction of a pressure medium causes non-uniform pressure on the ceramic article being consolidated resulting in distortion or loss of dimensional accuracy as noted in U.S. Pat. Nos. 4,501,718 and 4,539,175. This distortion or loss of dimensional accuracy increases the need for machining the article to its final dimensions and increases costs. The pressure medium increases the surface area or volume and this requires the higher pressures and/or heat for consolidation. Larger dies are necessary to accommodate the pressure medium resulting in increased machine size, labor and energy costs.
Other prior art techniques are shown in U.S. Pat. No. 4,041,123, which teaches a casted ceramic article that is made more dense by a pressure medium that requires heat and applied pressure to further increase density to the casted ceramic article. This technique can not produce a complex metal article in situ, but uses the heat and pressure operation to only further make dense a pre-consolidated article.
The prior art method shown in U.S. Pat. No. 4,547,337 discloses a method to consolidate a powder in which the powder to be consolidated is vacuumed in a hermetically sealed cylindrical container and is embedded in a glass material that becomes viscous at the desired temperature. Applied pressure deforms this glass pressure medium in turn applying pressure on the inner cylindrical container containing a powder which is then consolidated. It is mentioned that if a more intricately shaped article is required, the cylindrical inner container may be eliminated, and other materials of elastomers could be used to produce an intricately shaped rubber mold to encapsulate the powder metal which could transfer its shape to this powder metal under pressure producing an intricately shaped article. This method of manufacture requires a pressure medium that collapses, deforms or becomes viscous under heat and/or pressure to consolidate a powder metal. The nature of a pressure medium which becomes viscous or deforms under heat and/or pressure would transfer this deformation to the loose powder metal during consolidation, causing distortion and loss of dimensional accuracy, thus requiring machining the article to final its dimensions which increases costs. Additional steps of manufacturing, a cylindrical container and then requiring a glass material to be casted around this container also increases costs.
The use of the pressure medium step has the additional disadvantage of a requiring larger forging die to contain this medium, and, in turn, increases surface area or volume which requires higher heat and/or pressure to consolidate the metal powder. It also increases machine size, energy and labor costs. This prior art mentions the use of the elastomer tooling to apply pressure to the powder metal. The method has the disadvantage of deforming the loose metal powder at room temperature and would require additional steps to place the article in a pressure medium using heat and applied pressure for further consolidation to produce a high density article. The article would experience further deformation by this additional consolidation step requiring secondary machining to final its dimension which would increase costs.
A further process is shown in U.S. Pat. No. 4,389,362 which discloses a process for making a metal billet by encapsulating metal powder in a metal capsule, or as it more commonly known "metal can," and placing a pressure transmitting medium that becomes viscous at consolidation temperature between the can one and can two, which must be fabricated to encapsulate this pressure medium. A second pressure medium is required to compress can two. Heat and an applied pressure medium which makes more dense can two in turn makes more dense. The viscous pressure medium in turn makes more dense can one, which contains the powder metal. This is a method that requires numerous troublesome steps to consolidate a metal powder. First, fabricating a metal can which is usually done by sheet metal equipment to form a can required to contain a powder metal. This "can" requires embedding in an outer pressure medium requiring another can to be manufactured requiring another different pressure medium. This method requires two "cans" and two different pressure mediums and different pressure medium materials, medium one glass, medium two talc. This method cannot produce a complex shaped article in situ but requires multiple manufacturing steps of producing metal cans to contain either pressure medium or metal powder. The pressure medium that deforms or becomes viscous transfers that shape or deformity to the loose metal powder being consolidated. The additional outer pressure medium has interparticle friction which causes uneven pressure and causes additional distortion which is disclosed in U.S. Pat. Nos. 4,501,718 and 4,539,175. The distortion causes loss of dimensional accuracy and requires machining the article to its final dimensions which increases the steps and costs. These two pressure mediums increase volume and surface area which require larger dies, higher heat and/or pressure resulting in larger machinery required and higher energy and labor costs.